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SHARD_ROW_ID_BITS

This document introduces the SHARD_ROW_ID_BITS table attribute, which is used to set the number of bits of the shards after the implicit _tidb_rowid is sharded.

Concept

For the tables with a non-integer primary key or no primary key, TiDB uses an implicit auto-increment row ID. When a large number of INSERT operations are performed, the data is written into a single Region, causing a write hot spot.

To mitigate the hot spot issue, you can configure SHARD_ROW_ID_BITS. The row IDs are scattered and the data are written into multiple different Regions. But setting an overlarge value might lead to an excessively large number of RPC requests, which increases the CPU and network overheads.

  • SHARD_ROW_ID_BITS = 4 indicates 16 shards
  • SHARD_ROW_ID_BITS = 6 indicates 64 shards
  • SHARD_ROW_ID_BITS = 0 indicates the default 1 shard

Examples

  • CREATE TABLE: CREATE TABLE t (c int) SHARD_ROW_ID_BITS = 4;
  • ALTER TABLE: ALTER TABLE t SHARD_ROW_ID_BITS = 4;